Distillate fuels such as diesel fuels tend to exhibit reduced flow at reduced temperatures. This reduced flow affects the transport and use of the distillate fuels not only in the refinery but also in an internal combustion engine. If the distillate fuel is cooled to below a temperature at which solid formation begins to occur in the fuel, generally known as the cloud point (ASTM D 2500) or wax appearance point (ASTM D 3117), solids forming in the fuel will essentially prevent the flow of the fuel, plugging piping in the refinery, during transport of the fuel, and in inlet lines supplying an engine. Under low temperature conditions during consumption of the distillate fuel, as in a diesel engine, wax precipitation and gelation can cause the engine fuel filter to plug.
As used herein, distillate fuels encompass a range of fuel types, typically including but not limited to kerosene, intermediate distillates, lower volatility distillate gas oils, and higher viscosity distillates. Grades encompassed by the term include Grades No. 1-D, 2-D and 4-D for diesel fuels as defined in ASTM D 975, incorporated herein by reference. The distillate fuels are useful in a range of applications, including use in automotive diesel engines and in non-automotive applications under both varying and relatively constant speed and load conditions.
The cloud point of a fuel is the temperature at which a cloud of wax crystals first appears in a liquid when it is cooled under conditions prescribed in the test method as defined in ASTM D 2500, incorporated herein by reference. The cloud point behavior of a distillate fuel such as diesel fuel is a function of its composition. The fuel is comprised of a mixture of hydrocarbons including normal paraffins, branched paraffins, olefins, aromatics and other non-polar and polar compounds. As the diesel fuel temperature decreases at the refinery, during transport, or in a vehicle, one or more components of the fuel will tend to separate, or precipitate. The cloud point of the fuel is defined as the temperature at which the first waxes appear. The cloud point corresponds to an equilibrium state based on thermodynamic relationships which determine the solubility of paraffins in the diesel fuel.
Additives to decrease the cloud point, also known as cloud point depressants, have been used in fuels to delay the formation of solid wax crystals and thereby aid in enhancing the operability of the fuel. In addition, a cloud point depressant may also provide economic benefits in connection with the refining of the diesel fuel. To reach a particular cloud point specification, a certain amount of hydrocarbons in the kerosene boiling range are left in the diesel fuel fraction. A cloud point depressant will typically lower the cloud point by 2 to 3.degree. C. This lowering of the cloud point temperature by the depressant is known to compensate for the backing out of 20 to 30% of the kerosene fraction originally required to meet the cloud point specification.
The components of the diesel fuel having the lowest solubility tend to be the first to separate as solids from the fuel with decreasing temperature. Straight chain hydrocarbons, such as normal paraffins, generally have the lowest solubility in the diesel fuel, Generally, the paraffin crystals which separate from the diesel fuel appear as individual crystals. As more crystals form in the fuel, they tend to agglomerate and eventually reach a particle size which becomes visible to the eye and creates a cloudy appearance.
It is known to incorporate additives into diesel fuel to enhance the flow properties of the fuel at low temperatures. These additives are generally viewed as operating under either or both of two primary mechanisms. In the first, the additive molecules have a configuration which allow them to interact with the n-paraffin molecules at the growing ends of the paraffin crystals. The interacting additive molecules by steric effects act as a cap to prevent additional paraffin molecules from adding to the crystal, thereby limiting the length of the existing crystal.
In the second mechanism, the flow modifying additive may improve the flow properties of diesel fuel at low temperatures by functioning as a nucleator to promote the growth of smaller size crystals.
Additional, secondary, mechanisms involving the modification of wax properties in the fuel by incorporation of additives include, but are not limited to, dispersal of the wax in the fuel and solubilization of the wax in the fuel.
The range of available diesel fuels includes Grade No. 2-D, defined in ASTM D 975-90 as a general purpose, middle distillate fuel for automotive diesel engines, which is also suitable for use in non-automotive applications, especially in conditions of frequently varying speed and load. Certain of these Grade No. 2-D (No. 2) fuels may be classified as being hard to treat when using one or more additives to improve flow. A hard-to-treat diesel fuel is either unresponsive to a flow improving additive, or requires increased levels of one or more additives relative to a normal fuel to effect flow improvement.
Fuels in general, and diesel fuels in particular, are mixtures of hydrocarbons of different chemical types (i.e., paraffins, aromatics, olefins, etc.) wherein each type may be present in a range of molecular weights and carbon lengths. The cloud point temperature is a function of one or more properties of the fuel, the properties being attributable to the composition of the fuel. These properties include the paraffin concentration of the fuel, the molecular weight of the paraffins, and the chemical nature of the non-paraffin part of the fuel. For example, in the case of a hard-to-treat fuel the compositional properties which render a fuel hard to treat relative to normal fuels include a narrower wax distribution; the virtual absence of very high molecular weight waxes, or inordinately large amounts of very high molecular weight waxes; a higher total percentage of wax; and a higher average normal paraffin carbon number range. It is difficult to generate a single set of quantitative parameters which define a hard-to-treat fuel. Nevertheless, measured parameters which tend to identify a hard-to-treat middle distillate fuel include a temperature range of less than 100.degree. C. between the 20% distilled and 90% distilled temperatures (as determined by test method ASTM D 86 incorporated herein by reference), a temperature range less than 25.degree. C. between the 90% distilled temperature and the final boiling point (see ASTM D 86), and a final boiling point above or below the temperature range 3600 to 380.degree. C.
A fuel will cool to its cloud point generally in a static environment, but will also become cloudy in a dynamic environment such as a moving fuel tank at sufficiently low temperature. There continues to be a demand for additives which improve the cloud point characteristics of distillate fuels. Because additives are incorporated into the fuel to improve distinct characteristics of the fuel, it is possible that one additive may have an antagonistic effect on another additive. It is therefore desired that the cloud point additive not demonstrate an antagonism to the characteristics of the fuel as to one or more other properties, such as cold flow or wax anti-settling properties. Further, there remains a need for additive compositions which are capable of depressing the cloud point of hard-to-treat fuels.